Electrical isolation barriers can be identified in many industrial, medical and communication applications where it is necessary to electrically isolate one section of electronic circuitry from another electronic section. In this context isolation exists between two sections of electronic circuitry if a large magnitude voltage source, typically on the order of one thousand volts or more, connected between any two circuit nodes separated by the barrier causes less than a minimal amount of current flow, typically on the order of ten milliamperes or less, through the voltage source. An electrical isolation barrier must exist, for example, in communication circuitry which connects directly to the standard two-wire public switched telephone network and that is powered through a standard residential wall outlet. Specifically, in order to achieve regulatory compliance with Federal Communications Commission Part 68, which governs electrical connections to the telephone network in order to prevent network harm, an isolation barrier capable of withstanding 1000 volts nms at 60 Hz with no more than 10 milliamps current flow, must exist between circuitry directly connected to the two wire telephone network and circuitry directly connected to the residential wall outlet.
In many applications there exists an analog or continuous time varying signal on one side of the isolation barrier, and the information contained in that signal must be communicated across the isolation barrier. For example, common telephone network modulator/demodulator, or modem, circuitry powered by a residential wall outlet must typically transfer an analog signal with bandwidth of approximately 4 kilohertz across an isolation barrier for transmission over the two-wire, public switched telephone network. The isolation method and associated circuitry must provide this communication reliably and inexpensively. In this context, the transfer of information across the isolation barrier is considered reliable only if all of the following conditions apply: the isolating elements themselves do not significantly distort the signal information, the communication is substantially insensitive to or undisturbed by voltage signals and impedances that exist between the isolated circuitry sections and, finally, the communication is substantially insensitive to or undisturbed by noise sources in physical proximity to the isolating elements.
High voltage isolation barriers are commonly implemented by using magnetic fields, electric fields, or light. The corresponding signal communication elements are transformers, capacitors and opto-isolators. Transformers can provide high voltage isolation between primary and secondary windings, and also provide a high degree of rejection of lower voltage signals that exist across the barrier, since these signals appear as common mode in transformer isolated circuit applications. For these reasons, transformers have been commonly used to interface modem circuitry to the standard, two-wire telephone network. In modem circuitry, the signal transferred across the barrier is typically analog in nature, and signal communication across the barrier is supported in both directions by a single transformer. However, analog signal communication through a transformer is subject to low frequency bandwidth limitations, as well as distortion caused by core nonlinearities. Further disadvantages of transformers are their size, to weight and cost.
The distortion performance of transformer coupling can be improved while reducing the size and weight concerns by using smaller pulse transformers to transfer a digitally encoded version of the analog information signal across the isolation barrier, as disclosed in U.S. Pat. No. 5,369,666, "MODEM WITH DIGITAL ISOLATION" (incorporated herein by reference). However, two separate pulse transformers are disclosed for bidirectional communication with this technique, resulting in a cost disadvantage. Another disadvantage of transformer coupling is that additional isolation elements, such as relays and opto-isolators, are typically required to transfer control signal information, such as phone line hookswitch control and ring detect, across the isolation barrier, further increasing the cost and size of transformer-based isolation solutions.
Because of their lower cost, high voltage capacitors have also been commonly used for signal transfer in isolation system circuitry. Typically, the baseband or low frequency analog signal to be communicated across the isolation barrier is modulated to a higher frequency, where the capacitive isolation elements are more conductive. The receiving circuitry on the other side of the barrier demodulates the signal to recover the lower bandwidth signal of interest. For example, U.S. Pat. No. 5,500,895, "TELEPHONE ISOLATION DEVICE" (incorporated herein by reference) discloses a switching modulation scheme applied directly to the analog information signal for transmission across a capacitive isolation barrier. Similar switching circuitry on the receiving end of the barrier demodulates the signal to recover the analog information. The disadvantage of this technique is that the analog communication, although differential, is not robust. Mismatches in the differential components allow noise signals, which can capacitively couple into the isolation barrier, to easily corrupt both the amplitude and timing (or phase) of the analog modulated signal, resulting in unreliable communication across the barrier. Even with perfectly matched components, noise signals can couple preferentially into one side of the differential communication channel. This scheme also requires separate isolation components for control signals, such as hookswitch control and ring detect, which increase the cost and complexity of the solution.
The amplitude corruption concern can be eliminated by other modulation schemes, such as U.S. Pat. No. 4,292,595, "CAPACITANCE COUPLED ISOLATION AMPLIFIER AND METHOD," which discloses a pulse width modulation scheme; U.S. Pat. No. 4,835,486 "ISOLATION AMPLIFIER WITH PRECISE TIMING OF SIGNALS COUPLED ACROSS ISOLATION BARRIER," which discloses a voltage-to-frequency modulation scheme; and U.S. Pat. No. 4,843,339 "ISOLATION AMPLIFIER INCLUDING PRECISION VOLTAGE-TO-DUTY CYCLE CONVERTER AND LOW RIPPLE, HIGH BANDWIDTH CHARGE BALANCE DEMODULATOR," which discloses a voltage-to-duty cycle modulation scheme. (All of the above-referenced patents are incorporated herein by reference.) In these modulation schemes, the amplitude of the modulated signal carries no information and corruption of its value by noise does not interfere with accurate reception. Instead, the signal information to be communicated across the isolation barrier is encoded into voltage transitions that occur at precise moments in time. Because of this required timing precision, these modulation schemes remain analog in nature. Furthermore, since capacitively coupled noise can cause timing (or phase) errors of voltage transitions in addition to amplitude errors, these modulation schemes remain sensitive to noise interference at the isolation barrier.
Another method for communicating an analog information signal across an isolation barrier is described in the Silicon Systems, Inc. data sheet for product number SSI73D2950. (See related U.S. Pat. No. 5,500,894 for "TELEPHONE LINE INTERFACE WITH AC AND DC TRANSCONDUCTANCE LOOPS" and U.S. Pat. No. 5,602,912 for "TELEPHONE HYBRID CIRCUIT", both of which are incorporated herein by reference.) In this modem chipset, an analog signal with information to be communicated across an isolation barrier is converted to a digital format, with the amplitude of the digital signal restricted to standard digital logic levels. The digital signal is transmitted across the barrier by means of two, separate high voltage isolation capacitors. One capacitor is used to transfer the digital signal logic levels, while a separate capacitor is used to transmit a clock or timing synchronization signal across the barrier. The clock signal is used on the receiving side of the barrier as a timebase for analog signal recovery, and therefore requires a timing precision similar to that required by the analog modulation schemes. Consequently one disadvantage of this approach is that noise capacitively coupled at the isolation barrier can cause clock signal timing errors known as jitter, which corrupts the recovered analog signal and results in unreliable communication across the isolation barrier. Reliable signal communication is further compromised by the sensitivity of the single ended signal transfer to voltages that exist between the isolated circuit sections. Further disadvantages of the method described in this data sheet arc the extra costs and board space associated with other required isolating elements, including a separate high voltage isolation capacitor for the clock signal, another separate isolation capacitor for bidirectional communication, and opto-isolators and relays for communicating control information across the isolation barrier.
Opto-isolators are also commonly used for transferring information across a high voltage isolation barrier. Signal information is typically quantized to two levels, corresponding to an "on" or "off" state for the light emitting diode (LED) inside the opto-isolator. U.S. Pat. No. 5,287,107 "OPTICAL ISOLATION AMPLIFIER WITH SIGMA-DELTA MODULATION" (incorporated herein by reference) discloses a delta-sigma modulation scheme for two-level quantization of a baseband or low frequency signal, and subsequent communication across an isolation barrier through opto-isolators. Decoder and analog filtering circuits recover the baseband signal on the receiving side of the isolation barrier. As described, the modulation scheme encodes the signal information into on/off transitions of the LED at precise moments in time, thereby becoming susceptible to the same jitter (transition timing) sensitivity as the capacitive isolation amplifier modulation schemes.
Another example of signal transmission across an optical isolation barrier is disclosed in U.S. Pat. No. 4,901,275 "ANALOG DATA ACQUISITION APPARATUS AND METHOD PROVIDED WITH ELECTRO-OPTICAL ISOLATION" (incorporated herein by reference). In this disclosure, an analog-to-digital converter, or ADC, is used to convert several, multiplexed analog channels into digital format for transmission to a digital system. Opto-isolators are used to isolate the ADC from electrical noise generated in the digital system. Serial data transmission across the isolation barrier is synchronized by a clock signal that is passed through a separate opto-isolator. The ADC timebase or clock, however, is either generated on the analog side of the barrier or triggered by a software event on the digital side of the barrier. In either case, no mechanism is provided for jitter insensitive communication of the ADC clock, which is required for reliable signal reconstruction, across the isolation barrier. Some further disadvantages of optical isolation are that opto-isolators are typically more expensive than high voltage isolation capacitors, and they are unidirectional in nature, thereby requiring a plurality of opto-isolators to implement bidirectional communication.
Thus, there exists an unmet need for a reliable, accurate and inexpensive apparatus for effecting bidirectional communication of both analog signal information and control information across a high voltage isolation barrier, while avoiding the shortcomings of the prior art.
As mentioned above, one common application for electrical isolation barriers is for use in electrical connections to the standard two-wire public switched telephone network. FIG. 16 illustrates a typical prior art phone line termination circuit. FIG. 16 shows the standard two-wire public network lines, the TIP line 1602 and the RING line 1604. The TIP line 1602 and the RING line 1604 are conventionally connected to a diode bridge 1606. The diode bridge presents the proper polarity line signal to the hookswitch circuit 1608 independent of the TIP and RING polarity. The hookswitch circuit 1608 operates to "seize" or "collapse" the TIP and RING phone lines to allow the maximum loop current (I.sub.loop) that is available from the phone line to flow. The hookswitch circuit 1608 is coupled to electronic interface circuitry 1610. The electronic interface circuitry 1610 may contain a variety of devices and may be powered by the phone line. The electronic interface circuitry 1610 may also include an isolation barrier across which audio information may be transferred to the host powered circuitry 1616. A caller ID interface 1612 and a ringer interface 1614 may also be coupled between the TIP and RING lines and the host powered circuitry 1616. Both the caller ID interface 1612 and the ringer interface 1614 may also contain isolation barriers for coupling with the host powered circuitry 1616. The ringer interface 1614 operates to detect ring bursts on the phone line. Typical United States ring bursts are two second bursts of a 40 to 140 Vrms 15-68 Hz signal. The caller ID interface 1612 operates to extract the caller ID data which is embedded in the ring signal between the first and second ring burst. Generally the caller ID data is 1200/2200 Hz frequency shift keyed data. The end of ringing typically is indicated by the last ring burst being followed by a timeout period of approximately 5 seconds of no further ring bursts.
The circuits such as shown in FIG. 16 typically suffer from a number of problems. For example, typically the hookswitch circuits, caller ID interface, and ringer interface are all separate circuits. The hookswitch circuits and the caller ID interface are generally separated since the caller ID data is detected during on-hook conditions (ringing) when the hookswitch is off. Thus, separate circuitry in the caller ID interface is required to bypass the hookswitch and additional switch circuits in the Caller ID interface are opened during off-hook operation. The various switch circuits are typically implemented with external discrete high-voltage bipolar transistors including bipolar transistors dedicated for the hookswitch operation and bipolar transistors dedicated for the caller ID operation (see for example the Krypton Isolation, Inc. K.sup.2 930G DAA Data Sheet). Further, the ringer interface is often a non-linear network which would be unsuitable for detecting caller ID data and thus implemented through yet more separate circuitry. The use of these external components and separate circuits increases both costs and board space usage.
Another disadvantage of traditional interface techniques is that the ringer interface 1614 is generally formed from a combination of high voltage external components and opto-isolators. Further, the ringer interface may include integrated logic on the host side of the isolation barriers for performing burst detection, signal conditioning and timing functions. However, the use of high voltage external components and opto-isolators is undesirable due to costs. Moreover, all of the integrated ring detection circuits generally exist on the host side of the isolation barrier since generally only one-way communication exists with the opto-isolators.
Still another, disadvantage of traditional phone line interface techniques relates to the manner in which power supply voltages are obtained from the phone line signal. The phone line signal is a two wire system which provides both signal data and power by superimposing the signal data on a power supply voltage. A regulated voltage may be obtained from the power supply voltage and utilized for powering circuits such as analog to digital converters and digital to analog converters in the electronic interface device 1610. However, in order to maximize the regulated voltage, prior art techniques have attempted to minimize voltage drops across the diode bridge 1606 and the hookswitch 1608. To minimize these voltage drops, relays have been required for the hookswitch and special low voltage diodes have been utilized in the diode bridge (see for example the Siemans PSB4595 and PSB4596 Product Overview). Typically the relays may result in a voltage drop of almost zero and the low voltage diodes may be non-silicon diodes with voltage drops of 0.3-0.4 V. However, these components are undesirable due to increased costs.
Because of the disadvantages mentioned above and others, it is desirable to design an accurate yet a more efficient and cost effective phone line hookswitch interface, caller ID interface, and ringer interface. Moreover, it would be desirable to implement these interfaces in a system which includes an apparatus for effecting bidirectional communication across a high voltage isolation barrier.